The vision sensor or camera used in such a small scale 3D measuring machine has a very small depth of field; therefore usually a CAD model of the object to be gauged can be fed to and stored in the system as to know where to measure, and where holes, edges etc. should be so they can be avoided. Based on this a two dimensional scanning (2D scanning) of a more or less flat object with unknown small scale 2D geometry is possible, wherein more or less flat, means either completely flat, or with a very smooth height profile which could be tracked by serving the height of the sensor using a laser distance sensor.
Available measuring machines of these types are focused on different aspects: Beside the different scales (millimeter, micrometer, nanometer) they are adapted for, some are constructed in order to gain high speed measurements, others are constructed focused on a high precise measurement and others are created more to combine an acceptable speed with a reasonable preciseness and low cost.
With focus on a very fast measurement faster but more expensive vision measuring machines had been created by equipping the measuring machines with additional distance measuring units, like additional triangulation sensors or through-the-lens laser interferometers for distance measurements in particular allowing for faster and more efficient measurements in z direction. In those vision measuring machines usually more than one laser source is provided, wherein the emitted laser beams are distinguished by their different frequencies or signal codes. Using more than one laser source allows for determining simultaneously distances for as many points of the object surface as laser sources available. In another version of those measuring machines the various laser sources are used for creating a structured light pattern on the object surface for a large scaled, but less accurate measurement as it is known for 3D geometry acquisition of larger objects.
Another possibility for a very fast measuring machine, but having only one laser source for acquisition of very small dimensions of a workpiece is given in EP0270935A1. EP0270935A1 discloses a Coordinate Measuring Machine (CMM) based on a Delta Robot. The CMM has a fixed support table the object to be gauged is placed on. Further the CMM has a base plate provided to move in three coordinate directions x, y, z over the object to be gauged. The base plate is equipped with a camera for taking 2D images and with a laser distance measurement unit for distance measurement. The laser measurement unit has a laser emitter and a photosensitive detector for detecting a reflected laser light beam, emitted by the laser emitter and reflected by the object to be gauged. The distance measurement is used here in particular for focusing the camera. Therefore the emitted laser beam is emitted onto the object to be gauged at a location a bit ahead of the current position of the camera. The camera preferably has an optic with a small field of view for a high resolution. Further the CMM is provided with a global surveillance system having at least two stationary cameras observing the position of the workpiece and the movement of the base plate over the workpiece, wherein the stationary cameras having a large field of view. The focus of this measuring machine obviously is high speed measuring combined with reasonable preciseness. It can be driven in a high speed measurement mode with a quite good focusing of the camera, but the resulting system is complex and expensive.
Using measuring machines as described above, it is either possible to get a quite precise distance between the camera/laser source of the machine and of various points of an object by an expensive multi-laser source device or some cheaper a quite precise distance with respect to a small single spot of the object exposed to a single laser beam. Quite precise in this connection is always meant in relation to the absolute dimension of the object to be gauged and the distance between the object and the camera, the laser source and the laser detection unit, respectively. However, as already mentioned this quite precise information is only available very punctually and may—dependent on the properties of the reflecting surface—be afflicted with distinct uncertainties created by the noise detected together with the reflected laser light, particularly in case when laser speckles appear.